The present invention relates generally to a piezoelectric/electrostrictive element, a piezoelectric/electrostrictive device and a production method thereof, and more particularly to a laminated piezoelectric/electrostrictive element and a laminated piezoelectric/electrostrictive device made up of piezoelectric/electrostrictive layers and internal electrode layers laminated alternately, and a production method thereof.
In recent years, a variety of fields such as optics, magnetic recording, precision machining, and printing demand a displacement element for controlling, for example, the length or position of an optical path in the order of a submicron or vibrations precisely. As such a displacement element meeting the above demand, there is one employing displacement provided by a reverse piezoelectric effect or an electrostrictive effect taken place when the voltage is applied to a piezoelectric/electrostrictive material made of, for example, a ferroelectric substance.
Conventionally, as such a displacement element, a laminated piezoelectric element 100, as shown in FIG. 34, which is disclosed in Japanese Patent First Publication No. 4-309274 is known. The piezoelectric/electrostrictive element 100 includes, as shown in FIG. 34, a lamination 104 formed by laminating a plurality of piezoelectric ceramic layers 101 and electrode layers 102 alternately and a pair of electrically insulated external electrodes 104 and 105 which connect the electrode layers 102 alternately on opposed side surfaces of the laminate 103 and are so formed as to extend to upper and lower surfaces of the laminate 103. In the laminated piezoelectric element 100, ridges defined by the side surfaces and the upper and lower surfaces of the laminate 103 are rounded to an extent where the radius of curvature of the ridges exceeds half the thickness of the piezoelectric ceramic layers 101.
The production of the laminated piezoelectric element 100 shown in FIG. 34 is accomplished by first weighing and grinding raw material, mixing it with binder, and defoaming the mixture, after which the mixture is shaped into a sheet from which rectangular green sheets 101A are punched (which will be the piezoelectric ceramic layers 101 by baking). A conductive paste is printed over a given area of one surface of the green sheet 101A to form the electrode layer 102. Next, the green sheets 101A on which the electrode layers 102 are printed properly are, as shown in FIG. 35, laminated and bonded by pressure and cut as needed after which it is baked to produce the laminate 103 as shown in FIG. 36. As a result, the green sheets 101A are, as mentioned above, baked to produce the piezoelectric ceramic layers 101. In the laminate 103, arrangement positions of the electrode layers 102 are predetermined on a pair of opposed side surfaces thereof so that the electrode layers 102 may be exposed alternately. Afterwards, on given areas of upper and lower surfaces of the thus produced laminate 103, an external upper surface electrode 104A, and an external lower surface electrode 105A are formed. Next, on a pair of opposed side surfaces 106 and 107 to which the electrode layers 102 of the laminate 103 are exposed alternately, external side surface electrodes (thick film electrodes) 104B and 105B are formed to make the laminated piezoelectric element 100 show in FIG. 34. The external side surface electrode 104B is so formed as to connect with the external upper surface electrode 104A, while the external side surface electrode 105B is so formed as to connect with the external lower surface electrode 105A. As a method of forming the above mentioned external electrodes 104 and 105, there is a dipping method or an evaporation method.
FIG. 37 shows an actuator 200 utilizing the thus constructed laminated piezoelectric element 100. The actuator 200 has the laminated piezoelectric element 100 secured on a movable plate (diaphragm) 110 by an adhesive 111.
As another displacement element, a piezoelectric displacement element, as disclosed in Japanese Patent First Publication No. 63-295269, is known which is equipped with a plurality of opposed inner electrode layers in a ceramic thin plate exhibiting the piezoelectric effect. Corners that are boundaries of side surfaces and upper and lower surfaces of the ceramic thin plate are chamfered mechanically. On front and reverse surfaces and the opposed side surfaces of the ceramic thin plate, a pair of opposed surface electrodes connecting with internal electrode layers is so formed that the electrodes are electrically insulated from each other. The opposed surface electrodes are formed on the surfaces of the ceramic thin plate by a physical vapor deposition method such as a sputtering method or a vapor deposition method or a film forming method such as plating.
The laminated piezoelectric element 100, as shown in FIG. 34, has problems in that the possibility that edges of the green sheets 101A (shown in FIG. 35) are deformed, damaged, or broken by handling is high. Particularly, a thin piezoelectric element in which a total film thickness (thickness) of the laminate 103 is 100 xcexcm or less has a high possibility that the green sheets 101A are broken by handling. The conventional laminated piezoelectric element 100, thus, has a problem that the fabrication yield is low.
The piezoelectric displacement element, as disclosed in Japanese Patent First Publication No. 63-295269 is chamfered by mechanically cutting end portions of the ceramic thin film diagonally, which results in an increase in production processes. The mechanical cutting may also cause damage to the ceramic thin film.
The invention was made in order to solve the above problems. It is, thus, an object of the invention to provide a piezoelectric/electrostrictive element and a piezoelectric/electrostrictive device which are excellent in strength, shock resistance, handling, dimensional accuracy, positional accuracy, stability of element characteristics, and fabrication yield, and to provide a production method thereof.
In order to solve the above problems, the first feature of the invention lies in a piezoelectric/electrostrictive element including a substantially trapezoidal laminate having narrower and wider surfaces lying substantially in parallel to each other and first and second surfaces opposed to each other between the narrower and wider surfaces. The first and second surfaces are inclined at given angles with respect to one of the narrower and wider surfaces. The trapezoidal laminate is made up of a plurality of piezoelectric/electrostrictive layers and a plurality of internal electrodes, each of which is disposed between an adjacent two of the piezoelectric/electrostrictive layers. The internal electrodes are divided into a first and a second group, each of the first group internal electrodes lying over one of the second group internal electrodes through one of the piezoelectric/electrostrictive layers. A first external electrode is formed on the first surface of the laminate, and is coupled to the first group internal electrodes; and a second external electrode formed on the second surface of said laminate, said second external electrodes being coupled to the second group internal electrodes.
The thus constructed piezoelectric/electrostrictive element is of a substantially trapezoidal shape which decreases in width from one of the bottom surfaces to the other bottom surface, so that the angle which the slant surfaces of both sides make with the other bottom surface is obtuse, thus resulting in an increase in strength of a ridge portion (a corner) defined by the other bottom surface and the slant surfaces. Therefore, for example, when the other (narrower) bottom surface is secured on a movable plate (diaphragm), the breakage or damage of the ridge portion caused by an external force or vibrations of the piezoelectric/electrostrictive element itself is avoided. When the other bottom surface of the piezoelectric/electrostrictive element is secured on the movable plate (diaphragm) by adhesive, a recess-shaped (V-groove shaped) gap defined by the movable plate and the slant surfaces of both the sides of the piezoelectric/electrostrictive element can be filled with the adhesive, thereby resulting in a further increase in force (bonding strength) which secures the piezoelectric/electrostrictive element to the movable plate. The existence of the adhesive in the recess-shaped gap offers the effect of avoiding removal of the piezoelectric/electrostrictive element from the movable plate even if the stress arising from a difference in thermal expansion between the piezoelectric/electrostrictive element and the movable plate is produced.
The piezoelectric/electrostrictive layers are decreased in width gradually in one of the directions of lamination. Thus, for example, when the external electrode layers, the piezoelectric/electrostrictive layers, and the internal electrode layers are laminated in a given order, it is possible to pile up the piezoelectric/electrostrictive layers on a backing layer stably. Therefore, when the external electrode layers, the piezoelectric/electrostrictive layers, and the internal electrode layers are laminated by printing using, for example, a screen printing method, the printing is achieved easily since a lower one of the piezoelectric/electrostrictive layers has an area greater than that of an upper one of the piezoelectric/electrostrictive layers. The screen printing method makes it possible to apply, for example, via a conductive paste, the external electrode layers to the slant surface (a side surface portion) of the laminate.
Both the external electrode layers formed on the side surface portions extend along the wider surface of said laminate, thereby ensuring a joint area (a pad portion) which establishes a joint of wires for applying a drive voltage to the external electrode layers or wires for detecting a produced voltage, which facilitates connection of the wires. Particularly, as described above, when the narrower bottom surface of the laminate is secured on the movable plate, it is possible to bond the wires on a sufficient area of the wider bottom surface. The width of one of the external electrodes layers extending on the wider bottom surface of the laminate is greater, thereby allowing one of the external electrode layers to be used as a voltage applying electrode or a voltage detecting electrode.
Additionally, one of the surfaces of the piezoelectric/electrostrictive element may be formed by a piezoelectric/electrostrictive layer to increase a bonding strength using adhesive has the affinity to the piezoelectric/electrostrictive layers, for example, when the side of the piezoelectric/electrostrictive layer is bonded to the movable plate.
The second feature of the invention lies in a piezoelectric/electrostrictive device in which a piezoelectric/electrostrictive element includes a substantially trapezoidal laminate having narrower and wider surfaces lying substantially in parallel to each other and first and second surfaces opposed to each other between the narrower and wider surfaces. The first and second surfaces are inclined at given angles with respect to one of the narrower and wider surfaces. The trapezoidal laminate is made up of a plurality of piezoelectric/electrostrictive layers and a plurality of internal electrodes, each of which is disposed between an adjacent two of the piezoelectric/electrostrictive layers. The internal electrodes are divided into a first and a second group, each of the first group internal electrodes lying over one of the second group internal electrodes through one of the piezoelectric/electrostrictive layers. A first external electrode is formed on the first surface of the laminate, and is coupled to the first group internal electrodes. A second external electrode is formed on the second surface of said laminate, and is coupled to the second group internal electrodes. The piezoelectric/electrostrictive element is bonded to a surface of a movable plate on a side of the narrower surface of the laminate.
In the piezoelectric/electrostrictive device, the narrower bottom surface of the laminate is bonded to the surface of the movable plate, so that a corner portion having an obtuse angle defined by the narrower bottom surface and both side surface portions engages the movable plate. The corner having the obtuse angle will have strength greater than a corner having an acute angle or a right angle and offers the effect of increasing the durability such as the strength or shock resistance of the piezoelectric/electrostrictive device.
A gap (recess) formed by both the side surface portions of the piezoelectric/electrostrictive device and the movable plate defines a liquid sump of adhesive having flowability prior to solidification and works to absorb an excess or a lack of the adhesive caused by a variation in quantity of the applied adhesive or undulations of the movable plate and the piezoelectric/electrostrictive element. The application of a proper quantity of the adhesive to a suitable area of the movable plate will enable automatic alignment of the piezoelectric/electrostrictive element with a proper position with aid of an effect such as surface tension of the adhesive within the gap.
Additionally, filling the gap with the adhesive enables firm installation of the piezoelectric/electrostrictive element on the movable plate. If the adhesive with which the gap is filled keeps the elasticity after being solidified, it alleviates the stress arising from a difference in thermal expansion between the movable plate and the piezoelectric/electrostrictive element, thereby avoiding removable of the piezoelectric/electrostrictive element from the movable plate. Specifically, the filling of the gap defined by the side portions of the piezoelectric/electrostrictive element and the movable plate with the adhesive will restrict a reduction in strength to fix the piezoelectric/electrostrictive element even if the piezoelectric/electrostrictive element is decreased in size.
Further, the external electrode layers formed on both the side surface portions of the piezoelectric/electrostrictive element extend on the wider bottom surface of the laminate, thus providing a joint area sufficient for establishing connection of external wires to the external electrode layers.
The movable plate is made of a conductive material. One of the external electrode layers of the piezoelectric/electrostrictive element is connected to the movable plate, thereby increasing a wiring space of the other external electrode layer and facilitating ease of a connecting operation.
The third feature of the invention lies in a method of producing a piezoelectric/electrostrictive element including the following steps:
a first step of preparing a ceramic substrate having a given width;
a second step of forming a laminate on the ceramic substrate, the laminate being made up of first and second portions laid to overlap one another;
a third step of baking the ceramic substrate and the laminate at a given temperature; and
a fourth step of removing the laminate from the ceramic substrate;
the first portion of the laminate is formed using the following steps:
printing a first electrode layer and a second electrode layer on the ceramic substrate which are disposed at a given interval away from one another;
forming a piezoelectric/electrostrictive layer using a piezoelectric/electrostrictive paste on the first and second electrode layers so as to cover portions of the first and second electrode layers other than edge portions thereof lying outward in a widthwise direction of the ceramic substrate; and
forming a first electrode layer on an upper surface and a side surface of the piezoelectric/electrostrictive layer so as to establish an electric connection only with the first electrode layer lying immediately beneath the first electrode layer formed in this step.
The second portion of the laminate is formed by performing the following set of steps a given number of times, which include:
forming a piezoelectric/electrostrictive layer using a piezoelectric/electrostrictive paste on an uppermost one of the first electrode layers, the piezoelectric/electrostrictive layer formed in this step having a width smaller than that of the piezoelectric/electrostrictive layer lying immediately beneath the piezoelectric/electrostrictive layer formed in this step;
forming a second electrode layer on an upper surface and a side surface of an uppermost one of the piezoelectric/electrostrictive layers so as to establish an electric connection only with the second electrode layer lying immediately beneath the second electrode layer formed in this step;
forming a piezoelectric/electrostrictive layer using a piezoelectric/electrostrictive paste on an uppermost one of the second electrode layers, the piezoelectric/electrostrictive layer formed in this step having a width smaller than that of the piezoelectric/electrostrictive layer lying immediately beneath the piezoelectric/electrostrictive layer formed in this step; and
forming a first electrode layer on an upper surface and a side surface of an uppermost one of the piezoelectric/electrostrictive layers so as to establish an electric connection only with the first electrode layer lying immediately beneath the first electrode layer formed in this step.
In the method of producing the thus constructed piezoelectric/electrostrictive element according to the third feature, it is possible to pile up the piezoelectric/electrostrictive layers by printing so that areas thereof decrease gradually, thus resulting in ease of manufacture. The piezoelectric/electrostrictive layers, the first electrode material layer, and the second electrode material layer may be formed using a printing method, thus allowing a piezoelectric/electrostrictive element to be produced which is higher in dimensional accuracy, positional accuracy, less susceptible to adverse effects, such shifting during transportation and deformation caused by the transportation, and eliminating the need for process of transporting and piling up the piezoelectric/electrostrictive layers, which avoids breakage or damage of the piezoelectric/electrostrictive layers caused by handling thereof.
The formation of portions which become continuous external side surface electrodes on both sides of the laminate is achieved in sequence by repeating printing of the first and second electrode material layers, thus eliminating the need for a process of forming additional external side surface electrodes.
Further, a film which disappears upon baking of the laminate is preformed on a ceramic substrate used in producing the piezoelectric/electrostrictive element, thereby resulting in easy removal of the piezoelectric/electrostrictive element from the ceramic substrate when baked.
In this invention, the external side surface electrode which defines an outermost contour of the piezoelectric/electrostrictive element as viewed in a plane may be formed by printing with high positional accuracy. For example, when the piezoelectric/electrostrictive element is positioned by positioning pins to mount the piezoelectric/electrostrictive element on a movable plate, it is possible to arrange the piezoelectric electrostrictive element with high positional accuracy.
The fourth feature of the invention lies in a method of producing a piezoelectric/electrostrictive device in which a piezoelectric/electrostrictive element includes a substantially trapezoidal laminate having narrower and wider surfaces lying substantially in parallel to one another and first and second surfaces opposed to one another between the narrower and wider surfaces. The first and second surfaces are inclined at given angles with respect to one of the narrower and wider surfaces. The trapezoidal laminate is made up of a plurality of piezoelectric/electrostrictive layers and a plurality of internal electrodes, each of which is disposed between an adjacent two of the piezoelectric/electrostrictive layers. The internal electrodes are broken up into a first and a second group, each of the first group internal electrodes lying over one of the second group internal electrodes through one of the piezoelectric/electrostrictive layers. A first external electrode is formed on the first surface of the laminate and is coupled to the first group internal electrodes. A second external electrode is formed on the second surface of the laminate and is coupled to the second group internal electrodes. The piezoelectric/electrostrictive element is bonded to a surface of a movable plate by an adhesive.
In the method of producing the thus constructed piezoelectric/electrostrictive device, the narrower bottom surface side of the laminate is bonded to the movable plate through the adhesive, thus facilitating ease of filling a gap (recess) defined by slants of both side surfaces of the piezoelectric/electrostrictive element and the movable plate with the adhesive. This allows the gap to be filled with the adhesive in quantity suitable for the size of the gap, thus ensuring the bonding strength. When piezoelectric/electrostrictive elements are joined to each other, gaps (recesses) are also formed by side surface portions of the piezoelectric/electrostrictive elements, thus offering a similar effect of increasing the bonding strength.
The narrower bottom surface of the piezoelectric/electrostrictive element is bonded to the movable plate, so that angles which the side surface portions of the piezoelectric/electrostrictive element make with the movable plate will be obtuse, thus providing the effect of avoiding local breakage or damage of the piezoelectric/electrostrictive element. The same is true for a case where piezoelectric/electrostrictive elements are bonded at narrower bottom surfaces to one another.
Automatic positioning of the piezoelectric/electrostrictive element is achieved by setting a coefficient of viscosity of the adhesive applied on the surface of the movable plate to a given value to enable the filling with adhesive of the gaps (recesses) defined by the slants of the side surfaces of the piezoelectric/electrostrictive element and the movable plate.